<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Vocabulary</title>
<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="../align.html" title="Chapter 3. Boost.Align">
<link rel="prev" href="reference.html" title="Reference">
<link rel="next" href="compatibility.html" title="Compatibility">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td>
<td align="center"><a href="../../../index.html">Home</a></td>
<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="reference.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../align.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="compatibility.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="align.vocabulary"></a><a class="link" href="vocabulary.html" title="Vocabulary">Vocabulary</a>
</h2></div></div></div>
<h4>
<a name="align.vocabulary.h0"></a>
      <span class="phrase"><a name="align.vocabulary.basic_align"></a></span><a class="link" href="vocabulary.html#align.vocabulary.basic_align">[basic.align]</a>
    </h4>
<p>
      Object types have <span class="emphasis"><em>alignment requirements</em></span> which place restrictions
      on the addresses at which an object of that type may be allocated. An <span class="emphasis"><em>alignment</em></span>
      is an implementation-defined integer value representing the number of bytes
      between successive addresses at which a given object can be allocated. An object
      type imposes an alignment requirement on every object of that type; stricter
      alignment can be requested using the alignment specifier.
    </p>
<p>
      A <span class="emphasis"><em>fundamental alignment</em></span> is represented by an alignment
      less than or equal to the greatest alignment supported by the implementation
      in all contexts, which is equal to <code class="computeroutput"><span class="keyword">alignof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">max_align_t</span><span class="special">)</span></code>.
      The alignment required for a type might be different when it is used as the
      type of a complete object and when it is used as the type of a subobject. [<span class="emphasis"><em>Example:</em></span>
    </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
          <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">B</span>
          <span class="special">{</span> <span class="keyword">long</span>
          <span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> <span class="special">};</span></code>
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">D</span>
          <span class="special">:</span> <span class="keyword">virtual</span>
          <span class="identifier">B</span> <span class="special">{</span>
          <span class="keyword">char</span> <span class="identifier">c</span><span class="special">;</span> <span class="special">};</span></code>
        </li>
</ol></div>
<p>
      When <code class="computeroutput"><span class="identifier">D</span></code> is the type of a complete
      object, it will have a subobject of type <code class="computeroutput"><span class="identifier">B</span></code>,
      so it must be aligned appropriately for a <code class="computeroutput"><span class="keyword">long</span>
      <span class="keyword">double</span></code>. If <code class="computeroutput"><span class="identifier">D</span></code>
      appears as a subobject of another object that also has <code class="computeroutput"><span class="identifier">B</span></code>
      as a virtual base class, the <code class="computeroutput"><span class="identifier">B</span></code>
      subobject might be part of a different subobject, reducing the alignment requirements
      on the <code class="computeroutput"><span class="identifier">D</span></code> subobject. —<span class="emphasis"><em>end
      example</em></span>] The result of the <code class="computeroutput"><span class="keyword">alignof</span></code>
      operator reflects the alignment requirement of the type in the complete-object
      case.
    </p>
<p>
      An <span class="emphasis"><em>extended alignment</em></span> is represented by an alignment greater
      than <code class="computeroutput"><span class="keyword">alignof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">max_align_t</span><span class="special">)</span></code>. It is implementation-defined whether any
      extended alignments are supported and the contexts in which they are supported.
      A type having an extended alignment requirement is an <span class="emphasis"><em>over-aligned
      type</em></span>. [<span class="emphasis"><em>Note:</em></span> Every over-aligned type is or
      contains a class type to which extended alignment applies (possibly through
      a non-static data member). —<span class="emphasis"><em>end note</em></span>]
    </p>
<p>
      Alignments are represented as values of the type <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>.
      Valid alignments include only those values returned by an <code class="computeroutput"><span class="keyword">alignof</span></code>
      expression for the fundamental types plus an additional implementation-defined
      set of values, which may be empty. Every alignment value shall be a non-negative
      integral power of two.
    </p>
<p>
      Alignments have an order from <span class="emphasis"><em>weaker</em></span> to <span class="emphasis"><em>stronger</em></span>
      or <span class="emphasis"><em>stricter</em></span> alignments. Stricter alignments have larger
      alignment values. An address that satisfies an alignment requirement also satisfies
      any weaker valid alignment requirement.
    </p>
<p>
      The alignment requirement of a complete type can be queried using an <code class="computeroutput"><span class="keyword">alignof</span></code> expression. Furthermore, the types
      <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">signed</span>
      <span class="keyword">char</span></code>, and <code class="computeroutput"><span class="keyword">unsigned</span>
      <span class="keyword">char</span></code> shall have the weakest alignment
      requirement. [<span class="emphasis"><em>Note:</em></span> This enables the character types to
      be used as the underlying type for an aligned memory area. —<span class="emphasis"><em>end
      note</em></span>]
    </p>
<p>
      Comparing alignments is meaningful and provides the obvious results:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Two alignments are equal when their numeric values are equal.
        </li>
<li class="listitem">
          Two alignments are different when their numeric values are not equal.
        </li>
<li class="listitem">
          When an alignment is larger than another it represents a stricter alignment.
        </li>
</ul></div>
<p>
      [<span class="emphasis"><em>Note:</em></span> The runtime pointer alignment function can be used
      to obtain an aligned pointer within a buffer; the aligned-storage templates
      in the library can be used to obtain aligned storage. —<span class="emphasis"><em>end
      note</em></span>]
    </p>
<p>
      If a request for a specific extended alignment in a specific context is not
      supported by an implementation, the program is ill-formed. Additionally, a
      request for runtime allocation of dynamic storage for which the requested alignment
      cannot be honored shall be treated as an allocation failure.
    </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2014-2017 Glen
      Joseph Fernandes<p>
        Distributed under the Boost Software License, Version 1.0.
      </p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="reference.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../align.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="compatibility.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
